Garritan Interactive Principles of Orchestration by Nikolay Rimsky-Korsakov is the place where I went to begin studying orchestration. This offers the free “Interactive Edition of Principles of Orchestration based upon Nikolay Rimsky-Korsakov’s celebrated text”.
This provides easy to follow tutorials on how to use, mix, and match all the orchestral instruments with animated conductor score examples in each lesson.
I read through the entire course before I started doing anything with orchestration and I took a lot of notes. I intend to revisit the tutorials and see them again with a more experienced perspective.